1. Field of the Invention
This invention relates to a metal gasket used to seal a narrow space between opposed surfaces in a multi-cylinder engine and formed by interposing an intermediate plate between bead base plates comprising two elastic metal plates provided with beads along holes.
1. Description of the Prior Art
The structural members of an existing engine, such as a cylinder head and a cylinder block which are made of an aluminum alloy have small weight but a low rigidity, so that, while the engine is operated, the relative displacement occurring between these structural members tends to become large. Therefore, a metal gasket sealing a narrow space between the opposed surfaces of the two structural members is made of a metal material provided with beads on the portions thereof which are in the vicinity of the circumferences of through holes therein which are made correspondingly to cylinder bores, i.e. combustion chambers, and passages for water and oil.
A conventional metal gasket disclosed in Japanese Patent Laid-Open No. 293363/1988 is as shown in FIG. 10 disposed between a cylinder head 15 and a cylinder block 16, and includes two bead base plates 21, 22 comprising elastic metal plates on which annular beads 23, 24 are formed along the circumferences of combustion chamber holes 20, two intermediate plates 25, 26 disposed between the bead base plates, and spacer members 27 disposed between the intermediate plates 25, 26. In this metal gasket, the spacer members 27 are held between the regions of the two intermediate plates 25, 26 which are closer to the combustion chamber holes 20 than to the regions thereof which contact the beads 23, 24. Accordingly, one additional engaged surface exists between the two intermediate plates 25, 26 due to the provision of the spacer members 27. Moreover, the spacer members 27 have to be fixed by welding, and this causes the processing cost to increase, the increasing of the accuracy of the thickness of compensating portions to be substantially hampered, and the sealability of the metal gasket to decrease.
The metal gasket disclosed in Japanese Patent Laid-Open No. 65367/1989 is formed as shown in FIG. 11 by laminating two intermediate plates 35, 36 between two bead base plates 31, 32 comprising elastic metal plates on which annular beads 33, 34 the ridge portions of which are opposed to each other are formed along the circumferences of combustion chamber holes 30, and embracing a combustion chamber hole-side edge portion 37 of one intermediate plate 35 in that 38 of the other intermediate plate 36 so as to form a grommet-like structure and provide stepped portion on both surfaces of the intermediate plates, whereby a bent portion 39 for offsetting the irregularity of a narrow clearance between two opposed fixing surfaces. In this metal gasket, an edge portion 37 of one intermediate plate 35 is embraced in a grommet-like structure in an opposed edge portion 38 of the other intermediate plate 36, and substantially identical stepped portions are formed on both surfaces of the intermediate plates 35, 36. When the intermediate plates are held between the bead base plates 31, 32 on both sides thereof and compressed in a fully buckled state between the cylinder head 15 and the cylinder block 16, the grommet-like embraced folded portion comes to work as a shim for the beads 33, 34, and the stress amplitude of the bead base plates 31, 32 decreases. However, due to a difference between the operations of the upper and lower bead base plates 31, 32, bending stress and stress amplitude occur in the folded portion. Consequently, cracks and permanent set in fatigue occur in the bent portion 39 as well as in the bead base plates 31, 32.
In the conventional metal gaskets, the compensating portion in the circumferential edge of a combustion chamber hole and having the function of preventing the total compression of the beads on the two bead base plates, and the function of offsetting the irregularity of the opposed fixing surfaces occurring in a gasket tightening operation requires two intermediate plates basically. The compensating portion mentioned above is formed by using an additional member, such as a spacer or by a folded portion formed on the metal plates constituting the intermediate plates, so that increased manufacturing steps and quality control are required. This necessarily causes the manufacturing cost to increase.